--- Quote from: MECH_ENG on November 21, 2012, 09:28:13 PM ---[...] since I'm using analog control with 50' of ethernet line, what would be the best way to provide 5 volts to the potentiometers? Should I provide the 5 V from a source at the surface or from the 5 V pin on the Arduino aboard the ROV via a strand on the ethernet line?

--- End quote ---What potentiometers and what exactly do they control?

MECH_ENG:
The potentiometers are inputs to the Arduino board for speed and direction control of the thrusters and robot arm (servo driven). We plan to use the analog 3 axis joystick shown below.

--- Quote from: MECH_ENG on November 23, 2012, 04:41:39 PM ---The potentiometers are inputs to the Arduino board for speed and direction control of the thrusters and robot arm (servo driven). We plan to use the analog 3 axis joystick [...]

--- End quote ---That is a poor solution.The best would be to have eg. one of the cheap and very small Arduino compatible boards right at the stick, supply it and read it locally and then send a serial (digital) signal down the tether - a synchronous serial signal (i.e. Clock and Data) will probably be the easiest solution.

If you were to have 50' of wire between the stick and the A/D-lines to read it, you'd have nothing but trouble. Exactly how much is hard to say, as I cannot find any info on the potentiometer resistance (a bad sign).

The pots are likely between 50kOhm and 500kOhm and the A/D-C inputs are higher impedance - probably 1MOhm or more (see datasheet). This would result in fairly random values read at the receiving end, as the tether would act like an antenna and pick up anything from strong radio signals (broadcast and narrow-band), as well as any noise from switching electricity in any form, within a certain distance.

It's hard to ruin a digital signal with noise, but if it happens, sending data by a current signal (as opposed to a voltage signal) is easy and will cure any tendency to pick up noise.

MECH_ENG:
The tether will be an ethernet line not sure if that would change your answer, if not could you point me in the right direction for digital control?

Soeren:
Hi,

--- Quote from: MECH_ENG on November 23, 2012, 07:48:38 PM ---The tether will be an ethernet line not sure if that would change your answer,

--- End quote ---Still the same answer on the joystick control, but I wouldn't try to feed 18V at up to 20A through a Cat-5 cable.

The resistance of a single conductor in a Cat-5 varies from ~0.07 Ohm/m to over 1.2 Ohm/m (depending on make and quality). 50' is 15.24m and the length of the current path is twice that (two wires + and -).30.5m will be between 2.135 Ohm to 36.6 Ohm.2 thrusters (18V/4A each) equals a load of 2.25 Ohm.So...With the best Cat-5, the voltage over the thrusters will be: 18V * 2.25 Ohm / (2.25 Ohm + 2.135 Ohm) = 9.24V